Real will host Osasuna at the Santiago Bernabeu for their opening game of the new LaLiga season.
Speaking today in his media preview, Xabi was asked about new signing Mastantuono: "The first time I spoke to him, I was truly struck by his personality. He was still only 17 years old and very self-confident.
"He wasn't afraid of taking the step to come to Real Madrid. You can see his maturity, his desire, his quality. He's going to integrate very quickly into the team.
"He could get minutes tomorrow. He'll bring quality, energy, is committed defensively, and has good momentum. I've seen that he has that drive and that Argentine competitive spirit. He's good with the final pass. He's at a very good level."
Asked about Mastanuono declaring Barcelona legend Lionel Messi as the "best in the world", Xabi shrugged: "I can see why it makes sense that his favorite player is Messi. Being Argentine, it's no surprise to me."
Xabi was also asked about unsettled Brazil midfielder Rodrygo Goes.
"There are a lot of rumours during the summer. He's been doing well. I'm counting on everyone, and I want them to be committed to and for the team and to be at 100%. That's what worries me and what's occupying me right now."
We're all excited
Meamwhile, on his expectations for tomorrow night and the season overall, Xabi admits he is excited ahead of the kickoff.
He continued: "We're all approaching it with excitement, enthusiasm, and eager to get the season underway. It's been a short but intense two weeks, and we're already looking forward to stepping into the Bernabéu. We want to start well. The first game is always important. Tomorrow is the day.
"We're making progress. It's hard to measure. We certainly still have a few more to go. But we already introduced the main ideas and fundamentals at the Club World Cup, and we've worked hard on them these past two weeks. The feeling is good.
"I'm counting on everyone here, and I want everyone to be at 100%. We need that. I'm counting on everyone right now."
Xabi also commented on how Vinicius Jr and Kylian Mbappe have been adjusting to his playing system.
He added, "It's a matter of all the lines being aware of it. It's important for the forwards not to get caught out on the turns, but also that when we attack, the defensive line pushes. We want to function as a team.
"That's going to help us improve at all distances, both when we defend and when we attack. It's very important to be aware of them today. I think we made a step forward in the World Cup. It's something we're very aware of and have been working on."